This invention relates to sift-proof cartons and the method of manufacturing same, and, more particularly, to a carton having shortened end flaps and the method of sealing the end flaps with foamed hot melt adhesive to provide a sift-proof seal.
Hot melt thermoplastic adhesives are commonly used in applications such as packaging and cartoning where the quick setting time of this type of adhesive is advantageous. At the high operating speeds of commercial cartoning machines, the use of chemical or cold adhesives has decreased because of the relatively long setting time required for such adhesives. Hot melt adhesive applied to the flaps of a carton sets relatively quickly and substantially reduces the time in which compressive forces must be applied to the flaps while the adhesive bonds as compared to cold glue.
Despite the improvement over cold adhesives, hot melt thermoplastic adhesives also present problems in packaging and cartoning applications. One of the most common problems with hot melt adhesives is that of compressing the adhesive after application so as to obtain sufficient surface contact between the adhesive and adhered substrate to achieve a good bond. The relatively high viscosity, high surface tension, and quick setting time of hot melt adhesives all combine to prevent the adhesive from spreading over a large surface area when the adhesive is applied as a liquid to the substrate. Instead of spreading, the liquid sets up as a thick bead on the structure. Even when quickly compressed between two flaps of a carton, the adhesive has been found difficult to spread. In most instances, when the two flaps which have been adhered together are pulled apart, the bond breaks the adhesive-to-substrate interface. This means that in order to increase the strength of the bond, the area of the interface or surface contact between the adhesive and substrate must be increased.
As described in detail in U.S. Pat. No. 4,059,466, assigned to the assignee of this invention, it has been discovered that the adhesive strength of a bond achieved with a given quantity of hot melt adhesive may be appreciably improved if the adhesive is applied as a cellular foam rather than as a conventional non-foamed adhesive. A method of making and applying foamed hot melt adhesive is described in detail in U.S. Pat. No. 4,059,466. The increased bonding strength of the foamed adhesive results at least in part from the fact that the foamed adhesive may be spread over at least twice the area as the same adhesive in the non-foamed state under the same compressive conditions. Since the strength of the bond is a function of the area wetted or covered by the adhesive, foaming of adhesive results in a bond approximately twice as strong as the same quantity of adhesive unfoamed. Expressed another way, the same bond strength may be achieved with approximately half the quantity of foamed adhesive as unfoamed adhesive because of the much larger area wetted or covered by the unfoamed adhesive under the same compressive conditions.
So-called sift-proof cartons are usually fabricated by applying a block C or block U-shaped pattern of adhesive to the end flap structure of a four-sided carton to form a continuous line or bead of adhesive along the carton edges to eliminate any minute channels or openings through which granular material in the carton could leak. In one type of four-sided carton, for example, the four flaps at each end of the carton include a pair of opposed minor flaps, and an inner major flap and outer major flap which are each formed with a width equal to the depth of the carton. In sealing the end flaps of such four-sided, sift-proof carton, the opposed minor flaps are first folded inwardly toward the center of the carton. The two major flaps are placed in an open or spread position to receive hot melt adhesive. Usually, at least one strip or ribbon of hot melt adhesive is applied to each end of the inner major flap, transverse to its fold line. The outer major flap receives a block C or a block U-shaped strip of hot melt adhesive including a strip at each end perpendicular to its fold line, and a third strip extending longitudinally between the outer strips along the leading edge of the flap generally parallel to its fold line.
The sealing operation is completed by first folding the inner major flap onto the minor flaps beneath. The outer major flap is then folded over the inner major flap, and its U-shaped strip of adhesive contacts the ends and center portion of the exposed surface of the inner major flap beneath. Both the inner and outer major flaps extend across the entire depth of the carton. One example of this method of forming a sift-proof seal at the end flaps of a carton is shown in U.S. Pat. No. 3,831,342.
Sift-proof cartons of the type described above require a substantial amount of cold setting or hot melt adhesive to obtain the desired bond strength and to create a continuous barrier of adhesive between the flaps where product could leak out of the carton. In addition, both of the major flaps in cartons such as disclosed in U.S. Pat. No. 3,831,342 are formed with a width equal to the depth of the carton so that they extend across the entire bottom or top of the carton when folded. This is required to ensure that a continuous bead or barrier of adhesive is formed at the ends of the carton between the outer major flap and the inner major flap.
Another type of sift-proof carton employs shortened or economy inner and outer major flaps to save on carton material. These sift-proof cartons are the same as that shown in U.S. Pat. No. 3,831,342 except the inner and outer major flaps extend only part way across the top or bottom of the box instead of all the way across.
In sealing the end structure of a four-sided, sift-proof carton with economy major flaps, the opposed minor flaps are first folded inwardly toward the center of the carton. The two major flaps are placed in an open or spread position to receive hot melt adhesive. A strip or ribbon of hot melt adhesive is applied to each end of the inner major flap, transverse to its fold line, which is then folded onto the exposed surfaces of the minor flaps. When folded onto the minor flaps, the inner major flap forms a gap or space between its leading edge and the fold line of the outer major flap because the inner major flap extends only part way across the carton. In order to form a sift-proof seal, the gap or space between the leading edge of the inner major flap and fold line of the outer major flap overlying the minor flaps must be filled with adhesive.
It has been the practice in the prior art to form a sift-proof seal in cartons having economy major flaps to dispense a large quantity of hot melt adhesive on the ends of the outer major flap transverse to its fold line so that when the outer major flap is folded into position the hot melt adhesive fills the entire gap overlying the minor flaps between the leading edge of the inner major flap and the fold line of the outer major flap.
The problem with sift-proof seals of the type described above for cartons having economy flaps is that a large quantity of hot melt adhesive must be used to insure that a continuous, sift-proof seal is created in the gap overlying the minor flaps. This is due to the fact that hot melt adhesive has high viscosity and high surface tension which limits its spreadability, as discussed above. In using large quantities of hot melt adhesive, some of the adhesive is squeezed out from between the flaps when the outer major flap is folded in place. This creates either a sloppy looking seal or a seal which requires a further operation to remove the excess adhesive squeezed from underneath the flaps. Such an additional, adhesive removing operation adds to the cost of fabricating sift-proof seals of this type.
One other type of sift-proof seal is a so-called "delta seal" which is widely used to seal sugar bags and similar containers. The flaps of four-sided containers with delta seals include opposed rectangular-shaped flaps and opposed delta or triangular-shaped flaps. In forming a sift-proof container with this flap configuration, one of the rectangular-shaped flaps is folded toward the center of the container and a block C or block U-shaped strip of adhesive is applied to its exposed surface. The other rectangular flap is then folded toward the center of the container onto the adhesive placed on the exposed surface of the first rectangular flap. Each triangular flap receives a strip of adhesive along its outer edge, and the triangular flaps are then folded over the rectangular flap in sequence one after the other. An example of a delta seal for a sugar bag which is sealed by foamed hot melt adhesive is shown in U.S. Pat. No. 4,156,398.
One problem with delta seals is that it is difficult to apply a U-shaped ribbon of adhesive material to the rectangular flap, and then separate strips of adhesive along the angled edges of each triangular flap, in a high speed production run. In addition, each flap of a delta seal must be folded in sequence which further increases the complexity of forming such seals at high rates of speed.